The present invention relates to an osteoimplant for use in the repair, replacement and/or augmentation of various portions of animal or human skeletal systems, to a method for manufacturing the osteoimplant and to a method of using the osteoimplant. More particularly, this invention relates to an osteogenic osteoimplant which provides mechanical or structural support to a bone repair site.
Shaped or cut bone segments have been used extensively to solve various medical problems in human and animal orthopaedic surgical practice, and their application has also extended to the field of cosmetic and reconstructive surgery, dental reconstructive surgery, and other medical fields involving surgery of hard tissues. The use of autograft bone (where the patient provides the source), allograft bone (where another individual of the same species provides the source) or xenograft bone (where another individual of a different species provides the source) is well known in both human and veterinary medicine. In particular, transplanted bone is known to provide support, promote healing, fill bony cavities, separate bony elements (such as vertebral bodies), promote fusion (where bones are induced to grow together into a single, solid mass), or stabilize the sites of fractures. More recently, processed bone has been developed into shapes for use in new surgical applications, or as new materials for implants that were historically made of non-biologically derived materials.
Bone grafting applications are differentiated by the requirements of the skeletal site. Certain applications require a xe2x80x9cstructural graftxe2x80x9d in which one role of the graft is to provide mechanical or structural support to the site. Such grafts contain a substantial portion of mineralized bone tissue to provide the strength needed for load-bearing. The graft may also have beneficial biological properties, such as incorporation into the skeleton, osteoinduction, osteoconduction, or angiogenesis.
Structural grafts are conventionally made by processing, and then cutting or otherwise shaping cortical bones collected for transplant purposes. The range of bone grafts that might be thus prepared is limited by the size and shape limitations of the bone TISSUE from which the bone graft originated. Certain clinically desirable shapes and sizes of grafts may thus be unattainable by the cutting and shaping processes, due to the dimensional limitations of the bone. For some shapes they may also be available only in limited amounts, due to the large variations inherent in the human or animal donor source populations.
Many structural allografts are never fully incorporated by remodeling and replacement with host tissue due, in part, to the difficulty with which the host""s blood supply may penetrate cortical bone, and partly to the poor osteoinductivity of nondemineralized bone. To the extent that the implant is incorporated and replaced by living host bone tissue, the body can then recognize and repair damage, thus eliminating failure by fatigue. In applications where the mechanical load-bearing requirements of the graft are challenging, lack of replacement by host bone tissue may compromise the graft by subjecting it to repeated loading and cumulative unrepaired damage (mechanical fatigue) within the implant material. Thus, it is highly desirable that the graft have the capacity to support load initially, and be capable of gradually transferring this load to the host bone tissue as it remodels the implant.
It is an object of the present invention to provide an osteoimplant possessing sufficient strength in a body fluid environment to enable the osteoimplant to bear loads.
It is a further object of the present invention to provide a load-bearing osteoimplant which contains pores or cavities which permit the osteoimplant to be properly revascularized and incorporated by the host.
It is yet a further object of the present invention to provide a load-bearing osteoimplant which is osteogenic and thereby promotes new host bone tissue formation within and around the osteoimplant.
It is yet an even further object of the invention to provide a load-bearing osteoimplant which supports load initially and is capable of gradually transferring this load to the host bone tissue as it remodels the osteoimplant.
It is yet an even further object of the invention to provide a method for fabricating an osteoimplant which meets the foregoing objectives.
It is yet an even further object of the present invention to provide a method which enables the fabrication of osteoimplants of any size and/or shape.
It is yet an even further object of the present invention to provide a method for preparing osteoimplants which is not limited by constraints imposed by the shape and size of the original bone tissue from which the osteoimplants are derived.
These and further objects of the invention are obtained by a load-bearing osteoimplant which comprises a shaped, compressed composition of bone particles. The osteoimplant possesses a bulk density of greater than about 0.7 g/cm3 and a wet compressive strength of at least about 3 MPa. The osteoimplant of this invention is fabricated by the method which comprises providing a composition comprising bone particles optionally in combination with one or more biocompatible components and applying compressive force of greater than about 1000 psi to the composition to provide a load-bearing osteoimplant.
The bone particles utilized in the fabrication of the osteoimplant of this invention are selected from the group consisting of nondemineralized bone particles, demineralized bone particles, and combinations thereof. The bone particles are remodeled and replaced by new host bone as incorporation of the osteoimplant progresses in vivo. As described more fully hereinbelow, bone particles can be fully demineralized by removing substantially all of the inorganic mineral content of the bone particles, can be partially demineralized by removing a significant amount, but less than all, of the inorganic mineral content of the bone particles, or can be only superficially demineralized by removing a minor amount of the inorganic mineral content of the bone particles.
The term xe2x80x9cdemineralizedxe2x80x9d as applied to the bone particles utilized in the practice of the present invention is intended to cover all bone particles which have had some portion of their original mineral content removed by a demineralization process.
Nondemineralized bone particles provide strength to the osteoimplant and allow it to initially support load. Demineralized bone particles induce new bone formation at the site of the demineralized bone and permit adjustment of the overall mechanical properties of the osteoimplant. The osteoimplant of this invention optionally includes additional biocompatible component(s) such as wetting agents, biocompatible binders, fillers, fibers, plasticizers, biostatic/biocidal agents, surface active agents, bioactive agents, and the like.
The term xe2x80x9costeoimplantxe2x80x9d herein is utilized in its broadest sense and is not intended to be limited to any particular shapes, sizes, configurations or applications.
The term xe2x80x9cshapedxe2x80x9d as applied to the osteoimplant herein refers to a determined or regular form or configuration, in contrast to an indeterminate or vague form or configuration (as in the case of a limp or other solid mass of no special form) and is characteristic of such materials as sheets, plates, disks, cores, pins, screws, tubes, teeth, bones, portion of bone, wedges, cylinders, threaded cylinders, and the like.
The phrase xe2x80x9cwet compressive strengthxe2x80x9d as utilized herein refers to the compressive strength of the osteoimplant after the osteoimplant has been immersed in physiological saline (water containing 0.9 g NaCl/100 ml water) for a minimum of 12 hours and a maximum of 24 hours. Compressive strength is a well known measurement of mechanical strength and is measured using the procedure described herein.
The term xe2x80x9costeogenicxe2x80x9d as applied to the osteoimplant of this invention shall be understood as referring to the ability of the osteoimplant to enhance or accelerate the ingrowth of new bone tissue by one or more mechanisms such as osteogenesis, osteoconduction and/or osteoinduction.
The term xe2x80x9cincorporationxe2x80x9d utilized herein refers to the biological mechanism whereby host tissue gradually removes portions of the osteoimplant of the invention and replaces those removed portions with native host bone tissue while maintaining strength. This phenomenon is also known in the scientific literature as xe2x80x9ccreeping substitutionxe2x80x9d. The term xe2x80x9cincorporationxe2x80x9d utilized herein shall be understood as embracing what is known by those skilled in the art as xe2x80x9ccreeping substitutionxe2x80x9d.